1. Field of the Invention
The present invention relates to a process for molding a semi-rigid integral skin foam (ISF), wherein the semi-rigid ISF comprises an integral skin (self-skin) region having a low degree of foaming and a core region having a high degree of foaming. The present invention further relates to a polyurethane formulation for molding such a foam, which is suitable for reaction injection molding (RIM) of a molded product having an ISF, such as a covering or pad for a steering wheel, an instrument panel, a console box lid, a glove box lid, a headrest, an armrest, or an air spoiler of a motor vehicle.
2. Description of Related Art
A conventional ISF has been molded by injecting a polyurethane formulation containing freon into a mold cavity and by expanding freon by the heat generated by a urethane-forming reaction. The use of freons has been, however, prohibited or restricted because of their effects on the environment.
Therefore, the applicant has previously proposed a novel process for molding an ISF without using any freon, as disclosed in Japanese Patent Application Laid-Open No. 226313/1992. According to that invention, the process comprises evacuating a mold cavity, injecting a polyurethane formulation into the cavity, and expanding the formulation so that it flows and fills the cavity. The polyurethane formulation contains 0.1 to 0.6 parts by weight of water for 100 parts by weight of a polyol component. It was considered preferable to evacuate the mold cavity to 100 Torr or below if the amount of water is 0.1 parts by weight, and 400 Torr or below if the amount of water is 0.6 parts by weight.
When the process mentioned above was carried out by varying the amount of water in the polyurethane formulation and the degree of vacuum in the mold cavity, it was possible to obtain different degrees of foaming and thereby mold an ISF having cores which differed in density from one another. However, when the mold cavity was evacuated to 100 Torr or below to form a core having a low density of 0.6 g/cm3or less, it was impossible to make products having a good appearance, as such products looked depressed by shrinkage of the core or skin.
The present inventors also tried to form a low density core by increasing the water amount, or by introducing air bubbles by air loading. In either event, however, it was impossible to make products having a good appearance, because of an increase of defects, such as bulging and voids, on the core and skin, although the core was of low density.
As a result of further research, however, the present inventors have found that a polyurethane formulation containing another particular substance can be used to make a product having a core density as low as not exceeding about 0.6 g/cm3, and yet presenting a good appearance not spoiled by defects, such as shrinkage, bulging or voids.
It is, therefore, an object of this invention to form an ISF having a core of low density and presenting a good appearance not spoiled by defects, such as shrinkage, bulging or voids.
This object is attained by a process for molding a semi-rigid ISF having a low foamed integral skin layer and a highly foamed core, which comprises the steps of evacuating a mold cavity to 400 Torr or below, injecting into the cavity a polyurethane formulation comprising a polyol component, an isocyanate component, and at least one substance selected from the group consisting of formic acid, an amine salt of formic acid and an amine salt of boric acid, while not containing any freon, and foaming the polyurethane formulation to fill the cavity and thereby form the ISF.
The term xe2x80x9cpolyol componentxe2x80x9d means xe2x80x9ca composition comprising at least one polyolxe2x80x9d, while the term xe2x80x9cisocyanate componentxe2x80x9d means xe2x80x9ca composition comprising at least one polyisocyanatexe2x80x9d. xe2x80x9cFreonxe2x80x9d means xe2x80x9cblowing agents that comprise halogen atomsxe2x80x9d.
A cavity pressure of 400 Torr or below is essential for forming a core of low density, and a pressure of 100 Torr or below is preferable for ensuring a core density of 0.6 g/cm3 or less.
This invention also relates to a polyurethane formulation for a semi-rigid ISF having a low foamed integral skin layer and a highly foamed core, which comprises a polyol component, an isocyanate component, and at least one substance selected from the group consisting of formic acid, an amine salt of formic acid and an amine salt of boric acid, while not containing any freon.
In either event, the polyurethane formulation preferably has a water content not exceeding one part, and more preferably not exceeding 0.6 part, by weight relative to 100 parts by weight of polyol component so as to ensure the manufacture of a product having no defect such as bulging or voids. The amount of water content as defined includes both the water which is absorbed inevitably from the environment (normally in the range of 0.1 to 0.15 part by weight), and what has been added to it.
The polyurethane formulation is foamed mainly by the rapid bumping of the occluded (absorbed) gases therein at a reduced pressure if it has a low water content (for example, below 0.1 part by weight). The occluded gases include slight amounts of gases existing in the polyurethane formulation from the beginning, and a small amount of air entrained or dissolved within it during its circulation. Its foaming is also caused to some extent by a water-isocyanate reaction as described below.
If it has a high water content (such as, 0.1 part by weight or more), the polyurethane formulation""s foaming is caused mainly by the expansion at a reduced pressure of CO2 bubbles formed by the reaction of water and an isocyanate in the polyurethane formulation, which is promoted by the heat of a urethane reaction. Its foaming is, however, also caused to some extent physically by the boiling of water (having its boiling point lowered at a reduced pressure) and its vaporization, and by the bumping of the occluded gases.
In either event, the polyurethane formulation at the center of the mold cavity, which is away from the wall surface of the mold cavity, forms a core having a low density. The polyurethane formulation close to the wall surface of the mold cavity is easily debubbled at a reduced pressure, because the reaction heat dissipates through the wall surface of the mold cavity to thereby delay its urethane reaction and thickening. This results in the formation of a skin having a high density (or having a low degree of foaming).
The pressure in the mold cavity (vacuum degree) is 400 Torr or below for obtaining a core of low density, and a pressure of 100 Torr or below is preferable.
Examples of the amine salts of formic acid which can be used for the purpose of this invention are a salt of formic acid and bis (2-dimethylaminoethyl) ether (BDAEE), a salt of formic acid and triethylenediamine (TEDA), a quaternary ammonium salt of formic acid and a salt of formic acid and 1,8-diaza-bicyclo (5,4,0) undecene-7 (DBU). A mixture of two or more such salts can also be employed.
A salt of boric acid and BDAEE is an example of the amine salts of boric acid which can be employed.
At least one substance selected from the group consisting of formic acid, an amine salt of formic acid and an amine salt of boric acid is preferably employed in the amount of 0.03 to 5.0 parts by weight per 100 parts by weight of polyol component, though its amount may depend on which is employed, and the foaming degree required of the core. The use of a smaller amount may not bring about a satisfactory result in preventing the occurrence of defects as stated above, while the use of a larger amount is uneconomical, as such leads to hardly any better result.
Further objects of this invention will become evident upon an understanding of the illustrative embodiments described below. Various advantages not specifically referred to herein but within the scope of the instant invention will occur to one skilled in the art upon practice of the presently disclosed invention. The following examples and embodiments are illustrated and not seen to limit the scope of the invention.